One portion of the solar spectrum comprises wavelengths of electromagnetic energy which range between about 290 and 3,000 nanometers (nm). This range may be divided into different regions, namely: (1) the ultraviolet region (290–400 nm), (2) the visible region (400–760 nm) and (3) the near-infrared region (>760 nm). The ultraviolet region has, moreover, been arbitrarily divided into three bands, referred to as the UVA, UVB and UVC bands.
The UVB band extends from 290 to 320 nm. It is the principal cause of the sunburn reaction and it is also the most effective in stimulating the tanning reaction in the skin. UVC radiation (200–290 nm) from the sun does not reach the surface of the earth, although one can encounter radiation in this range from artificial sources such as germicidal lamps and high and low pressure mercury arc lamps. For purposes of the present invention, however, protection against UVC radiation is generally not a major concern, i.e., in contrast to the dangers posed by UVA and UVB radiation. The UVA band, which extends from 320–400 nm, can also cause the tanning reaction. UVA radiation can also cause sunburns, but its capacity to do so is less than that of UVB radiation.
The amount of UVA radiation exposure, however, is increasing. This is due to the fact that most sunscreens effectively block only UVB radiation. As stated above, UVB radiation is more capable than UVA radiation of causing the tanning and burning reactions. Therefore, if one is using a sunscreen that blocks UVB radiation he/she will tend to stay in the sun for an extended period of time because the immediate effects of the sun tan/bum are not evident. The problem is that UVA is still penetrating the skin and although it is not causing any immediately obvious effects, it is causing long-term damage. Long-term hazards of ultraviolet radiation include premature aging of the skin. This condition is characterized by wrinkling and yellowing of the skin, along with other physical changes such as cracking, telangiectasis (spider vessels), solar keratoses (growths), ecchymoses (subcutaneous hemorrhagic lesions), and loss of elasticity (sagging).
In recent years, it has been well documented that UVA radiation, like UVB radiation, is harmful to the skin. In fact, current data reveal that solar radiation containing these wavelengths (A and B) is a contributing cause of skin cancer, which presently accounts for 30–40% of all new cancers each year. In the United States alone, 500,000 new cases of skin cancer will be reported this year and the number is expected to keep rising in the future. UVA radiation has been shown to promote skin cancer by inhibiting enzymes that repair cells damaged by UVB radiation. UVA radiation also penetrates more deeply into the skin than UVB radiation and causes changes in blood vessels and premature aging of the skin, thus adding to the damage produced by UVB rays. The goal of any sunscreen should thus be to protect the user from both UVA and UVB radiation with a minimum of side effects.
The “SPF” (Sun Protection Factor) is recognized as the ratio of the irradiation time required to elicit a minimum erythemal reaction (sunburn) on sunscreen protected skin using a solar simulator, to the irradiation time required to elicit the same minimum erythemal reaction (sunburn) on unprotected skin. This test is conducted under clinical conditions according to the procedure described in the Proposed Monograph for Sunscreen Containing Drug Products (hereafter referred to as the Proposed Monograph) published by the U.S. Food and Drug Administration (FDA) in the U.S. Federal Register, Vol. 43, Aug. 25, 1978, Part 2, pages 38206–38269, which is incorporated herein by reference. As used herein, the term “SPF” or Sun Protection Factor is defined in accordance with the definitions in the Proposed Monograph. This same publication also describes the clinical testing procedure mandated for determining whether sunscreen products are waterproof, water resistant and sweatproof. The labeled SPF values are generally recognized as being between 2 and 50. This is not meant to imply that SPF values greater than 50 are unachievable given the previous formulation technology. However, the amounts of sunscreen agents needed to achieve such high SPF values are usually cost prohibitive given current formulation technologies. The concentration of sunscreen agents needed to satisfy a waterproof designation are particularly high, because some of the agents are washed away in the test that measures SPF for a waterproof composition.
In general, the SPF number approximately corresponds to the multiple of time during which the properly applied sunscreen will prevent obvious reddening of the skin, over the exposure time that causes unprotected skin to exhibit reddening. Thus, a person should be able to remain in the sun without visible effects for eight times the usual unprotected duration, if an SPF 8 sunscreen formulation has been properly applied. Of course, the duration of unprotected exposure, which produces a visible effect on the skin, varies from one individual to another, due to differences in their skin cells. Currently popular are high-SPF “sunblocker” products, having SPF values of at least 30.
A sunscreen works on the surface of the skin to absorb UV radiation so that the harmful rays never enter the skin. Commercially available sunscreen products contain from about 3 to about 26% of one or more UV absorbing chemicals. When applied to the surface of the skin as a thin film, these chemicals act as a filter to diminish the penetration of UV radiation to the cells of the epidermis. These sunscreens are typically applied in a cream, oil, lotion, alcohol or gel vehicle and they are usually colorless because they do not contain any visible light-absorbing chemicals. The most widely used organic-based sunscreens contain, for example, paraminobenzoic acid (PABA), PABA esters (glyceryl PABA), amyldimethyl PABA and octyldimethyl PABA), benzophenones (oxybenzone and sulisobenzone), cinnamates (octylmethoxy cinnamate and cinoxate), salicylates (homomethyl salicylate), anthranilates such as menthyl anthranilate, 2-ethylhexyl-2-cyano-3,3-diphenyl acrylate, 2-phenyl benzimidazole-5-sulfonic acid, digalloyl trioleate, 3-(4-methyl benzylidene) camphor, 4-isopropyl dibenzoyl methane, butyl methoxy dibenzoyl methane, 2-ethyl-2-cyano-3,3′-diphenyl acrylate.
Common sun care products sold in today's market include oil-in-water emulsions incorporating stearic acid neutralized with triethanolaminc. The SPF values of such emulsions range from 2 to 50, and they commonly include ethylhexyl methoxycinnamate as the sunscreen agent. As the SPF of these formulations increases, they commonly contain ethylhexyl salicylate, homosalate, octocrylene and/or oxybenzone in addition to the ethylhexyl methoxycinnamate mentioned above. Alternatively, padimate O can be used in place of the ethylhexyl methoxycinnamate or the salicylates mentioned above. Dioxybenzone, avobenzone or menthyl anthranilate can be used in place of oxybenzone. If the product does not claim to be substantive to the skin (i.e., waterproof or water resistant), trolamine salicylate or DEA methoxycinnamate can be used in place of (or in combination with) the ethylhexyl methoxycinnamate, ethylhexyl salicylate or homosalate. Additionally, sulisobenzone may be used in such non-substantive formulations in place of oxybenzone. The Proposed Monograph lists 21 active ingredients that can be used individually or in combinations to achieve the desired product SPF.
U.S. Pat. No. 4,917,882 (“'882 patent”) discloses a gel-type sunscreen composition comprising about 1 and about 30 percent of sunscreen agent, between about 5 and about 25 percent polyethylene, and between about 20 and about 95 percent of a benzoate ester. The benzoate ester is important to the composition of the '882 patent because it provides a translucent, non-oily feeling, anhydrous vehicle which is well suited for carrying the sunscreen agents. In particular, the patent notes that benzoate esters do not interfere with the UV absorption of typical sunscreen agents and that compounds such as mineral oil have been found to shift the absorbance curve of agents such as Padimate O, thereby reducing their ability to absorb UV radiation in the erythemal region.
Sunscreen compositions in the form of creams and lotions are two-phase systems in which one of the phases is finely and uniformly dispersed within the other to form an emulsion. Mixing the two phases with an appropriate surfactant emulsifier, which also functions to stabilize the emulsion and initially makes the dispersion. The sun screening ingredients are usually solubilized in the oil phase, which is, most commonly, the internal or dispersed phase. However, after application and prolonged exposure of the skin to water, the surfactants actually facilitate the emulsifier removal of the product from the skin by causing the oils to re-emulsify.
Several approaches have been taken in an effort to solve this problem. One solution is to resort to water in oil emulsions. Because this type of emulsion is usually not sufficiently water soluble to dissolve or disperse in water, it therefore has little ability to re-emulsify oily material into water. Another technique used to reduce the re-emulsification of the sunscreen's internal “oily phase” is to use emulsifiers that lose their ability to emulsify once they have been applied to the skin. These are typified by the “polymeric” surfactants that rely upon a “sphere of hydration” to establish and maintain their water solubility. Once the hydration sphere is lost (due to drying when the product is applied to the skin), the polymeric surfactant loses its ability to emulsify and the oil phase along with the dissolved sunscreens stay on the skin.
Another way to eliminate re-emulsification is to simply not include any surfactants in the formulation. Sunscreen oils take this approach and do not show significant reductions in sun protection factors following bathing. However, sunscreen or sun tanning oils are considered to have an unpleasant, “greasy” feel and they can have an unfavorably high cost of goods as they are made entirely of oils. Creams and lotions, on the other hand, can contain 70% to 80% water and therefore have a concomitantly lower production cost, but they suffer from the re-emulsification problem described above.
The need exists, therefore, for multiphase, high SPF sunscreen compositions that provide adequate and prolonged protection against both UVB and UVA radiation even upon extended exposure to water. Ideally, such compositions would have a low production cost and would not evoke any unpleasant sensation upon application to the skin. Further, the need exists for multiphase sunscreen products in which the various phases are visibly distinct prior to application, thereby enhancing the cosmetic appeal of the products.